How is rust too low level for a desktop app? All the native ui toolkits are in C/C++ ...

Before I started programming in Rust I had similar concerns when I saw people pushing rust as backend language choice. However having written a non significant amount of rust it does really well at feeling a lot like ruby/python while still achieving high performance and low level access.

Is there any good/mature/sane library for UI in Rust? I'd love to use Rust for a desktop app I want to write.

This comes up in /r/rust periodically, and IIRC the answer is still pretty much "no". It is getting better, though, slowly. I'm on my phone and too lazy to find a link for you, but search /r/rust for discussions about GUI libraries/frameworks.

Integration with GTK is getting easier and nicer, but there's more work to do.

Does Nuklear fit your definition of sane UI library?

https://github.com/vurtun/nuklear

https://github.com/snuk182/nuklear-rust

That looks interesting, but I'm talking about something more like GTK, ie something that looks somewhat native and is easily portable across OSes.

Yeah but you are much more productive using them from a higher level language binding.

E.G: QT used from Python with PyQT.